A tap-changer is a selector switch which enables the selective electrical connection of one output with one of several contact sections. An on-load tap-changer has the ability to switch under load current. Typically the contact sections are arranged along a circular path wherein the selector contact is rotatable around a virtual axis in the center of the circular path. But also a selector contact which is moveable in axial direction along axially arranged contact sections is a suitable embodiment of an on-load tap-changer. On-load tap-changers are used to adapt the transmission ratio of a power transformer in distribution networks within certain limits.
In this case taps of the transformer winding are electrically connected with respective contact sections of a tap-changer, wherein a certain voltage difference in between the respective connector sections is present, which might be at around +/−10% of the rated voltage of the transformer over all taps. In case of a circular arrangement the connector sections are typically foreseen on the inner surface of an electric insulating hollow casing. The electrical insulation in between adjacent connector sections has to be foreseen to withstand a respective voltage difference. The highest voltage difference will rise in between first and last tap, which are adjacent due to the circular arrangement.
Oil transformers are known for voltages of 380 kV and higher and for a rated power of some 100 MVA for example. As an oil transformer itself also a tap changer for an oil transformer is flooded with the insulation medium oil, which enables a compacter design due to the reduced required insulation distance in between adjacent connector sections.
Dry transformers avoid the insulation medium oil due to reasons of improved security or easier handling. Dry transformers are typically known for voltages up to 36 kV or 72.5 kV with a rated power of for example some MVA. Due to this—at least in relation—not as high voltage the required insulation distances in between the different selector contacts within the tap-changer are not as high so that the size of a respective oil-free on-load tap-changer is still within acceptable limits. Anyhow, the rated voltage for dry transformers is expected to increase in the future, for example to 110 kV.
Disadvantageously within this state of the art is that the size of an oil-free tap-changer for dry transformers is exceeding an acceptable limit with increase of the rated voltage.